I would have loved to tell you how to overcome it, but the truth is I don't know. I can relate to the feeling of your brain not working and not being able to grasp or start even the simplest thing that used to come 'easier'. I'm constantly trying to push myself to work on the school project I still need to start knowing it's only a few more steps till summer break. It's the thought of all things that still need to be finished, especially this research paper, that make me feel overwhelmed, which makes sense since even just getting up is a big deal already. I don't know how to remove this wall of not being able to work on what used to be your interest/passion. It could be anything from needing some kind of structure to know exactly how and when to work on things, to maybe just needing some time off from school, to being depressed.

Also, does this feeling of no motivation happen in other situation as well, or just when it comes to your thesis? I think it's good to discuss this with someone. Do you have anyone near you or perhaps someone at school that you might be able to talk with or get help from? I really hope you will find some answers and manage to work on your thesis eventually.